2.

5 Climate and natural vegetation

Key questions
H What are the main characteristics of equatorial and hot desert climates?
H What are the factors that influence equatorial and desert climates?
H What are the characteristics of tropical rainforest and hot desert ecosystems?
H What are the causes and impacts of the deforestation of tropical rainforest?

that is higher than the seasonal differences in

temperature
» mainly convectional rainfall, which may fall on as
many as 250 days each year
» variable cloud cover — in the morning it may
be limited but, by the afternoon, towering
cumulonimbus clouds mark the start of the
convectional rains
» reduced heat loss at night due to the presence of
clouds — hence the diurnal range is less than in
hot desert areas
» high humidity (moisture in the atmosphere) —
relative humidities of 100 per cent are often
▲ Figure 2.96 Tropical rainforest, Sarawak reached in the late afternoon
» reduced wind speeds within the rainforest due to
Equatorial and hot desert the large numbers of trees present.

climates The data for Manaus in Brazil (Table 2.20) show that
the warmest months are September and October, with
The main characteristics of an equatorial climate include: a mean monthly temperature of 34°C. In contrast, all
» hot conditions — generally above 26°C — of the months from December to September share the
throughout the year mean minimum monthly temperature of 24°C. Thus
» high levels of rainfall, often over 2000 mm the mean annual temperature range is 10°C.
» a lack of seasons — the temperatures are high Rainfall in Manaus is high — nearly 2100 mm.
throughout the year There is a definite wet season between November
» a difference between daytime and night-time and May, whereas the months of June to October are
temperatures (known as the diurnal range) relatively dry.

Table 2.20 Climate data for Manaus

J F M A M J J A S O N D Mean/total
Temperature
Daily max (°C) 31 31 31 31 31 31 32 33 34 34 33 32 32
Daily min (°C) 24 24 24 24 24 24 24 24 24 25 25 24 24
Average monthly (°C) 28 28 28 27 28 28 28 29 29 29 29 28 28
Rainfall
Monthly total (mm) 278 278 300 287 193 99 61 41 62 112 165 220 2096
Sunshine
Sunshine (hours) 3.9 4.0 3.6 3.9 5.4 6.9 7.9 8.2 7.5 6.6 5.9 4.9 5.7

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 Factors affecting climate

Table 2.21 Climate data for Cairo

J F M A M J J A S O N D Mean/total
Temperature
Daily max (°C) 19 21 24 28 32 35 35 35 33 30 26 21 28
Daily min (°C) 9 9 12 14 18 20 22 22 20 18 14 10 16
Average monthly (°C) 14 15 18 21 25 28 29 28 26 24 20 16 22
Rainfall
Monthly total (mm) 4 4 3 1 2 1 0 0 1 1 3 7 27
Sunshine
Sunshine (hours) 6.9 8.4 8.7 9.7 10.5 11.9 11.7 11.3 10.4 9.4 8.3 6.4 9.5

In contrast, the main characteristics of hot desert

climates include: Factors affecting climate
Many factors affect the temperature of a place. These
» very hot days and cold nights, caused by the lack include latitude, distance from the sea, the nature
of cloud cover of nearby ocean currents, altitude, dominant winds,
» low and irregular amounts of rainfall, which lack cloud cover and aspect. Differences in pressure
any seasonal pattern systems also affect whether it rains or whether it
» low levels of humidity for much of the year is dry.
» warm, dry winds, sometimes causing sandstorms.
The data for Cairo (Table 2.21) show that the highest Latitude
mean monthly temperatures are between June and On a global scale latitude is the most important
August, when the temperature reaches 35°C. In factor determining temperature (Figure 2.97). Two
contrast, the lowest mean monthly temperature is factors affect the temperature: the angle of the
in January, reaching just 9°C. Thus the temperature overhead Sun and the thickness of the atmosphere.
range is 26°C. There is a seasonal pattern to Firstly, at the equator the overhead Sun is high
temperature, with the highest values in the summer in the sky, hence high-intensity insolation is
and lowest readings in the winter. Rainfall figures received. By contrast, at the poles the overhead
are very low — just 27 mm. Sunshine levels are lower Sun is low in the sky, hence the quantity of energy
during the months when there is more rain (winter received is low. Secondly, the thickness of the
months between November and March). In general, atmosphere affects temperature. Radiation has
sunshine levels are much higher in Cairo — 9.5 hours more atmosphere to pass through near the poles,
per day compared with 5.7 hours per day in Manaus. due to its low angle of approach. Hence more
energy is lost, scattered or reflected here than over
Activities equatorial areas, making temperatures lower over
the poles.
1 In which months is the average temperature in Cairo
Equatorial climates have high temperatures
higher than in Manaus?
2 How much rain falls in Manaus in April? throughout the year on account of their location.
3 In which months is the minimum temperature in They also receive high levels of rainfall due to the
Cairo higher than that in Manaus? How do you explain daily convection. Hot deserts are hot due to their
this? tropical location, but receive low rainfall for a variety
4 Describe the variations in monthly sunshine levels in of reasons, including the presence of the subtropical
Manaus.
5 Suggest why there is a link between sunshine levels
high-pressure belt.
and rainfall.
6 What is the mean monthly temperature range in
Manaus and Cairo in:
a July
b December?

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 2.5 Climate and natural vegetation

Latitude Distance from the sea

Near the poles insolation has Summer
more atmosphere to pass through Incoming heat
from the Sun
15°C (warm)
Coastal region
10 °C (cool) cooled by sea air
Land absorbs
Solar radiation Sea absorbs heat slowly heat quickly
(insolation)

Winter
Outgoing heat
from the Earth
0°C (cold)
Atmosphere Coastal region
5 °C (cool) warmed by sea air
Land loses
At the equator insolation is concentrated, but near the poles it is Sea loses heat slowly heat rapidly
dispersed over a wide area
▲ Figure 2.97 Factors that affect climate

Proximity to the sea winter. Areas that lie close to cold, upwelling ocean
currents, such as Namibia in Africa, may contain hot
The specific heat capacity is the amount of heat
deserts, such as the Namib desert. This is because
needed to raise the temperature of a body by 1°C.
the cold current cools the air above it, reducing the
Land heats and cools more quickly than water. It takes
amount of evaporation from the ocean, and producing
five times as much heat to raise the temperature of
dry conditions.
water by 1°C as it does to raise land temperatures.
Water also heats more slowly because: Altitude
» it is clear, so the Sun’s rays penetrate to great In general, air temperature decreases with increasing
depth, distributing energy over a wider area altitude. This is because air under the greater
» tides and currents cause the heat to be distributed pressure of lower altitudes is denser and therefore
further. warmer. As altitude increases, so the pressure on the
air is reduced and the air becomes cooler. The normal
Therefore, a greater volume of water is heated for
decrease of temperature with height is, on average,
every unit of energy than land, so water takes longer
10°C/km.
to heat up.
Distance from the sea therefore has an important Winds
influence on temperature. Water takes up heat and
The effects of wind on temperature depend on the
emits it much more slowly than the land. In mid
initial characteristics of the wind. In temperate
latitudes in winter air over sea is much warmer than
latitudes prevailing (dominant) winds from the land
over land, so onshore winds bring heat to the coastal
lower the winter temperatures, but raise them in
lands. By contrast, during the summer coastal areas
summer. This is because continental areas are very
remain much cooler than inland sites. Areas with a
hot in summer but very cold in winter. Prevailing
coastal influence are termed maritime or oceanic,
winds from the sea do the opposite — they lower the
whereas inland areas are called continental. Areas
summer temperatures and raise them in winter.
that are very far from the sea may be extremely arid,
such as parts of central North Africa. Cloud cover
Cloud cover decreases the amount of insolation
Ocean currents reaching the surface by reflecting some of it. Clouds
The effect of ocean currents on temperatures depends also reduce the amount of insolation leaving the
on whether the current is cold or warm. Warm surface by absorbing the radiation. If there is
currents from equatorial regions raise the temperature limited cloud then incoming shortwave radiation and
of polar areas (with the aid of prevailing westerly outgoing longwave radiation are at a maximum. This
winds). However, the effect is only noticeable in is the norm in many hot deserts.
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 Tropical rainforests

Pressure as farming and logging. The result is that rainforests

are disappearing and those that remain are not only
In low-pressure systems air is rising. Low pressure
smaller, but broken up into fragments.
produces rain as the air may rise high enough, cool,
condense and form clouds and rain. This can happen
in very warm areas, such as in equatorial areas, at Interesting note
mountain barriers and at weather fronts, when warm
Tropical rainforests cover 6 per cent of the world’s
air is forced over cold air. In contrast, where there
is high pressure air is sinking, and rain formation is land surface but hold 50 per cent of the world’s
prevented. The world’s great hot deserts are located species. The Amazon rainforest alone is home to
where there is high pressure caused by sinking air. 10 per cent of the world’s known species.

Activities Vegetation
1 How does latitude affect the amount of heat a place The vegetation is evergreen, enabling photosynthesis
receives? to take place all year round. This is possibly due to
2 Why are equatorial areas not getting any hotter, or the high temperatures all year, and the presence
polar areas any colder? of water throughout the year. The vegetation is
3 What is meant by the term ‘specific heat capacity’? layered, and the shape of the crowns varies at
4 Explain why temperature decreases with height.
5 Why is there a large temperature difference between
each layer (Figure 2.99). Species at the top of the
day and night in hot deserts, but not in equatorial canopy receive most of the sunlight, whereas species
areas? that are located near the forest floor are adapted
to darker conditions, and generally have a darker
pigment so as to photosynthesise at low light levels.
Tropical rainforests There is a great variety in the number of species
Evergreen tropical rainforests are located in in a rainforest — this is known as biodiversity.
equatorial areas, largely between 10°N and 10°S A rainforest may contain as many as 300 different
(Figure 2.98). There are, nevertheless, some areas species in a single hectare. Typical rainforest species
of rainforest that are found outside these areas, but include figs, teak, mahogany and yellow woods.
these tend to be more seasonal in nature. The main Tropical vegetation has many adaptations. Some
areas of rainforest include the Amazon rainforest in trees have leaves with drip-tips (Figure 2.100a),
Brazil, the Congo rainforest in central Africa, and which are designed to get rid of excess moisture. In
the Indonesian-Malaysian rainforests of Southeast contrast, other plants have saucer-shaped leaves in
Asia. There are many small fragments of rainforest, order to collect water. Pitcher plants have developed
such as those on the island of Madagascar and in an unusual means of getting their nutrients. Rather
the Caribbean. Tropical rainforests everywhere are than taking nutrients from the soil, they have
under increasing threat from human activities, such become carnivorous and get their nutrients from
insects and small frogs that are trapped inside the
pitcher (Figure 2.100b). This is one way of coping
Tropical rainforest with the very infertile soils of the rainforest. Other
plants are very tall. To prevent being blown over by
the wind, very large trees have developed buttress
roots that project out from the main trunk above
Tropic of Cancer the ground, which gives the plant extra leverage in
the wind.
Equator

Tropic of Capricorn
Rainforest animals
Although rainforests cover less than 6 per cent of
the Earth’s surface, they account for over 50 per
cent of all animal species on Earth. It is more than
▲ Figure 2.98 World distribution of tropical rainforests

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 2.5 Climate and natural vegetation

A
Wide-spaced
umbrella-shaped 45
crowns, straight
trunks and high 40 A Emergent (top)
branches tree canopy

Height above ground (m)

35

B 30
Medium-spaced
mop-shaped 25
crowns B Large trees of
middle layer
20
C
Densely packed
15
conical-shaped
C Lower tree
crowns
10 layer
D
Sparse 5 D Shrub/small tree
vegetation of layer
shrubs and E Ground vegetation
saplings
F Root layers F Root zone

▲ Figure 2.99 Vegetation structure of the tropical rainforest

a b which camouflages their fur in their arboreal

environment. Some animals have evolved to look
larger or scarier than they really are: the larvae
of the lobster moth look like scorpions, but are
defenceless. Many butterflies have designs that
look like large eyes on their wings, in order to
confuse potential predators. Some species, such as
the monarch butterfly, are poisonous, which helps
deter predators.

Soils
Rainforests are the most productive land-based
ecosystems. Rainforest soils are typically deep
due to the large amount of weathering that has
▲ Figure 2.100 Adaptations of rainforest plants: (a) drip-tip; taken place, and they are often red in colour, due
(b) pitcher plant to the large amounts of iron present in the soil.
Ironically, the soils of tropical rainforests are quite
likely that many rainforest species have not yet been infertile. This is because most of the nutrients
discovered — between 2010 and 2013 over 440 new in the rainforest are contained in the biomass
species of plant and animal were discovered in the (living matter). Nevertheless, there are some areas
Amazon alone. in which tropical soils may be more fertile: in
Many species, such as orang-utans, are arboreal floodplains and in volcanic areas the soils may be
(live entirely in the trees) and rarely come to the enriched by flooding or the weathering of fertile
ground where they would be easy prey for large lava flows.
carnivores. Top carnivores, such as jaguars, tigers The nutrient cycle is easily disrupted (Figure 2.101).
and leopards, are highly camouflaged. Many species Tropical rainforests have been described as ‘deserts
are highly camouflaged to avoid becoming prey, covered by trees’. Once the vegetation is removed,
such as stick insects and the Indian leaf butterfly. nutrients are quickly removed from the system,
Sloths are covered with a layer of green algae, creating infertile conditions and even deserts.

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 Impacts of deforestation of the tropical rainforest

Rainforests are found only in areas with over 1700 mm of rain and temperatures The links between climate, soils
of over 25 °C and vegetation are very strong

30 Tropical rainforest
Hot desert
temperature (°C)

25
Mean annual

Climate
20 Deciduous forest
15
10
5 Coniferous forest
0
0 500 1000 1500 2000 2500 3000 3500 4000 4500 Soil Vegetation
Mean annual rainfall (mm)

▲ Figure 2.101 Conditions required for the growth of rainforest, deciduous forest, coniferous forest and hot desert

industry. High rainfall totals, especially in hilly areas,

Activities favour the development of HEP, such as at Batang Ai
Study Figure 2.101. in Sarawak, Malaysia. Areas of rainforest have a long
1 What is the minimum temperature required for the history of commercial farming. Tropical hardwoods,
growth of tropical rainforests? such as teak and mahogony, are prized by furniture
2 What is the minimum amount of rainfall needed for a manufacturers. Mineral mining developments, such
tropical rainforest? as iron ore at Carajas in Brazil and ilminite on the
3 Suggest how a rainforest with a mean annual southeast coast of Madagascar, are also developed in
temperature of 30°C and an annual rainfall of
3500 mm might differ from one with a mean annual
some rainforest areas.
rainfall of 1700 mm and a mean annual temperature
of 25°C.
4 Suggest how the vegetation in Figure 2.100 is adapted
to conditions in the rainforest.
5 What is biodiversity? Suggest reasons why it may be
important to protect biodiversity.
6 Why are rainforests described as ‘deserts covered
with trees’?

Impacts of deforestation of
the tropical rainforest
About 200 million people live in areas that are or were
covered by tropical rainforests. These areas offer many ▲ Figure 2.102 Tropical rainforest along with shifting
cultivation — rice growing in Sarawak
advantages for human activities, such as farming,
hydroelectric power, tourism, fishing and food supply,
mineral development and forestry (Figure 2.102).
Rainforests also play a vital role in regulating the
world’s climate, and they account for 50 per cent of
the world’s plants and animals. They are vital, too, for
the protection of soil and water resources (Table 2.22).
The year-round growing season is very attractive
for farmers, although the poor quality of the soil
results in the land being farmed for only a few
years before the land is abandoned (Figure 2.103).
Nevertheless, large-scale plantations occur in areas
of tropical rainforest, producing crops such as palm ▲ Figure 2.103 Rainforest at Batang Ai affected by flooding,
oil, which is increasingly being used for the biofuels shifting cultivation and soil erosion
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 2.5 Climate and natural vegetation

Table 2.22 The value of tropical rainforests Other changes relate to climate. As deforestation
Industrial uses Ecological uses Subsistence uses progresses, there is a reduction in water that is re-
Charcoal Watershed Fuelwood and evaporated from the vegetation, hence the recycling
protection charcoal of water must diminish. Evaporation rates from
Saw logs
Flood and landslide Fodder for
savanna grasslands are estimated to be only about
Gums, resins and one-third of those of the tropical rainforest. Thus,
protection agriculture
oils
mean annual rainfall is reduced, and the seasonality
Soil erosion control Building poles
Pulpwood of rainfall increases.
Climate regulation, Pit sawing and saw
Plywood and
veneer
e.g. balancing
levels of carbon
milling Causes of deforestation in Brazil
Weaving materials There are five main causes of deforestation in Brazil:
Industrial dioxide and oxygen
and dyes
chemicals
Special woods and
Rearing silkworms » agricultural colonisation by landless migrants
Medicines ashes and speculative developers along highways and
and beekeeping
Genes for crops Fruit and nuts agricultural growth areas
Tourism » conversion of the forest to cattle pastures,
especially in eastern and southeastern Para and
northern Mato Grosso
There are a large number of effects of deforestation, » mining, for example the Greater Carajas Project in
including: southeastern Amazonia, which includes a 900 km
» disruption to the circulation and storage of railway and extensive deforestation to provide
nutrients charcoal to smelt the iron ore. Other threats from
» surface erosion and compaction of soils mining include the small-scale informal gold
» sandification mines, garimpeiros, causing localised deforestation
» increased flood levels and sediment content of rivers and contaminated water supplies
» climatic change » large-scale hydroelectric power schemes, such as
» loss of biodiversity. the Tucurui Dam on the Tocantins River
» forestry taking place in Para, Amazonas and
Deforestation disrupts the closed system of nutrient northern Mato Grosso.
cycling within tropical rainforests. Inorganic elements
are released through burning and are quickly flushed Deforestation in Brazil shows five main trends:
out of the system by the high-intensity rains. » It is a recent phenomenon.
Soil erosion is also associated with deforestation. » It has partly been promoted by government policies.
As a result of soil compaction, there is a decrease in » It has a wide range of causes.
infiltration, and an increase in overland runoff and » It includes new areas of deforestation, as well as
surface erosion. the extension of previously deforested areas.
Sandification is a process of selective erosion. » Land speculation and the granting of land titles
Raindrop impact washes away the finer particles to those who ‘occupy’ parts of the rainforest is a
of clay and humus, leaving behind the coarser and major cause of deforestation.
heavier sand. Evidence of sandification dates back to
the 1890s in Santarem, Rondonia.
As a result of the intense surface runoff and soil Activities
erosion, rivers have a higher flood peak and a shorter 1 Comment on the value of tropical rainforests to the
time lag. However, in the dry season river levels are human population.
2 Outline the main impacts of deforestation on the
lower, the rivers have greater turbidity (murkiness natural environment.
due to more sediment), an increased bed load, and 3 Explain the main causes of deforestation in Brazil.
carry more silt and clay in suspension. 4 Comment on the trends of deforestation in Brazil.

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 Impacts of deforestation of the tropical rainforest

Case study: Danum Valley Conservation Area, Malaysian Borneo

▲ Figure 2.104 Danum Valley Conservation Area

The Danum Valley Conservation Area (DVCA) contains commercial sectors are all represented. To the east of the
more than 120 mammal species, including 10 species of DVCA is the 30,000 hectare Innoprise-FACE Foundation
primate. The DVCA and surrounding forest is an important Rainforest Rehabilitation Project (INFAPRO), one of the
reservation for orang-utans. These forests are particularly largest forest rehabilitation projects in southeast Asia,
rich in other large mammals including the Asian elephant, which is replanting areas of heavily disturbed logged forest.
Malayan sun bear, clouded leopard, bearded pig and Because all areas of conservation and replanting are
several species of deer. The area also provides one of embedded within the larger commercial forest, the value of
the last refuges in Sabah for the critically endangered the whole area is greatly enhanced. Movement of animals
Sumatran rhino. Over 340 species of bird have been between forest areas is enabled and allows the continued
recorded at Danum, including the argus pheasant, Bulwer’s survival of some important and endangered Borneo
pheasant and seven species of pitta bird. animals such as the Sumatran rhino, the orang-utan
The DVCA covers 43,800 hectares, comprising almost and the Borneo elephant. In the late 1990s, a hotel was
entirely lowland dipterocarp forest (dipterocarps are established on the northeastern edge of the DVCA. It has
valuable hardwood trees). It is the largest expanse of established flourishing ecotourism in the area and exposed
pristine forest of this type remaining in Sabah, northeast this unique forest to a wider range of visitors than was
Borneo (Figure 2.104). previously possible. As well as raising revenue for the local
Until the late 1980s, the area was under threat from area, it has raised the international profile of the area as an
commercial logging. The establishment of a long-term important centre for conservation and research.
research programme between Yayasan Sabah and the
Royal Society in the UK created local awareness of the
Case study analysis
conservation value of the area and provided important 1 What was the main threat to the Danum Valley before
scientific information about the forest and what happens the late 1980s?
to it when it is disturbed through logging. Danum Valley 2 Why is the DVCA important for the conservation of
is controlled by a management committee containing species?
all the relevant local institutions — wildlife, forestry and 3 What are the main interest groups in the forest?

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 2.5 Climate and natural vegetation

Hot deserts woody. Annuals or ephemerals are plants that live for
a short time but these may form a dense covering of
The world’s hot deserts are largely found in
vegetation immediately after rain.
subtropical areas between 20° and 30° north and
Ephemerals evade drought. During the infrequent
south of the equator (Figure 2.105). The largest
wet periods they develop rapidly, producing a large
area of hot desert is the Sahara but there are other
number of flowers and fruits. These help produce
important deserts such as the Great Victoria Desert
seeds, which remain dormant in the ground until the
and Great Sandy Desert in Australia, the Kalahari and
next rains.
Namib deserts in southern Africa, the Atacama desert
Many plants are adapted to drought (Figure 2.106)
in South America, and the Arabian desert. The Gobi
— these are called xerophytes. Water loss is
desert in Mongolia and China lies outside the tropics
minimised in a number of ways:
and therefore is not a hot desert.
The main factors influencing the vegetation are » Leaf hairs reduce windspeed and therefore reduce
that it is hot throughout the year and there is low transpiration.
and unreliable rainfall (≤250 mm per year). » Thick waxy cuticles and the rolling-up or shedding
of leaves at the start of the dry season reduce
Vegetation water loss.
The vegetation responds to hot desert conditions » Some plants have the bulk of their biomass (living
in a number of ways. There are two main types of material) below the ground surface.
desert plant. Perennials (plants that grow over a » Others have very deep roots to reach the water
number of years) may be succulent (they store lots table.
of water), they are often small (to reduce water loss » In woody species the wood prevents the collapse
by evaporation and transpiration) and they may be of the plant even when the plant is wilting.

Sonoran
Desert

Sahara
Mohave
Desert

Atacama
Desert
Namib Great Sandy
Kalahari
Desert and Great Victoria
Desert
Deserts

▲ Figure 2.105 World distribution of hot deserts

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 Hot deserts

acacia thickets), spinifax in Australia (‘porcupine

grass’) and chanaral in Chile (spiny shrubs).
Soils in desert areas are very infertile. As a result
of the low rainfall there is little organic or moisture
content in the soil. The lack of chemical weathering
(largely due to the lack of moisture) means that
soils contain few nutrients.

Interesting note
Although it is a hot desert, the Sahara contains
some 300 plant species and around 70 animal
species.

Animals
Animals are adapted to living in the desert in a
number of ways (Figure 2.107). Different animals may:
» be nocturnal (active only at night) in order to
avoid the heat of the day
» use panting and/or have large ears help to reduce
body heat
» remain in underground burrows during the day
▲ Figure 2.106 Plant adaptations to hot desert environments » secrete highly concentrated uric acid in order to
reduce water loss
Vegetation from desert margins is often referred to » migrate during the hottest season to escape the heat
as scrub. Tropical scrub on the margins of hot deserts » adopt a strategy of long-term aestivation
includes acacias, cacti, succulents, tuberous-rooted (dormancy, or sleep), which ends only when
plants and herbaceous plants that only grow with moisture and temperature conditions become more
rain. Special types are mulga in Australia (dense favourable.

Kangaroo rat Fennec fox

Lives in southwest USA and Mexico; Lives in Sahara desert: weighs 1.5 kg
weighs 35–180 g and measures 100–200 mm and is 200 mm tall

Light-coloured coat reflects heat

Highly efficient kidneys which Reduces water lost by
Large ears with
concentrate urine and produce respiration by cooling
blood vessels
dry droppings exhaled air in nasal passage
close to the
surface to lose
body heat

Gets most of its

moisture from
its prey

Excretes
highly
concentrated
urine

Lives in burrows Does not Licks fur to Does not need to

during the day perspire keep cool drink – gets water
Lives in burrows during Soles of feet protected by
to avoid heat from its food
the day to avoid heat thick fur to run across hot sand
▲ Figure 2.107 Animal adaptations to hot desert environments
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 2.5 Climate and natural vegetation

Activities
1 Why is it difficult to live in a hot desert? 250 mm might differ from one in which the mean
2 Study Figure 2.101 on page 161, which shows the annual rainfall is 250 mm and the mean annual
conditions required for the growth of hot deserts. temperature is 20°C.
a What is the maximum rainfall in a hot desert, as 4 How have plants adapted to survive in the desert?
suggested by Figure 2.101? 5 How do animals survive in the desert?
b What is the range of mean annual temperatures in 6 Visit www.bbc.co.uk/nature/life/Camel to find out how
hot deserts? camels are adapted to living in hot deserts.
3 Suggest how a hot desert with a mean annual
temperature of 30°C and a mean annual rainfall of

Case study: The Sonoran desert

The Sonoran Desert is located in southern USA (southern There has been considerable human impact in
California and southern Arizona) and northern Mexico. the area. Some cities, notably Phoenix in Arizona,
Its vegetation includes the saguaro cactus, which can have expanded rapidly at the expense of the desert.
grow to a height of 15 m and live for up to 175 years. Its The increased demand for, and abstraction of,
ribbed stem expands as it fills with water during the water has lowered water tables. Mesquite bushes
winter wet season. Its stem also reduces wind speed and and cottonwood trees that were growing along
water loss from the plant, while sunken stomata reduce watercourses have died back. Road construction and
water loss. It has shallow roots to catch water from pipelines have affected the movement of mammals,
storms before it evaporates. Other species with similar and fenced highways have prevented pronghorn
adaptations include the prickly pear and barrel and antelopes, for example, from reaching water supplies.
hedgehog cacti. Off-road vehicles have compacted soils and made
The Palo Verde is a small, drought-tolerant tree. It loses them less able to hold water. Overgrazing by cattle
its leaves in the dry season, but its green bark allows it has removed more palatable species. Domesticated
to photosynthesise without leaves. Creosote bushes have animals have escaped into the wild, and reduced
small, dark leaves to reduce transpiration. Plant density grazing availability for wild mammals. The introduction
depends upon water availability. of exotic plant species, such as tamarisk, has
Soils in the Sonoran desert are typically thin, relatively displaced native species such as cottonwoods and
infertile and alkaline. Seasonal rains carry soluble desert willows. Removal of native species has speeded
salts down through the soil. However, during the dry up the spread of exotic species.
season they are drawn up to the surface by evaporation.
Concentrations may be toxic for some plants. In addition,
Case study analysis
flash flooding can compact the soil, leaving the surface 1 Where is the Sonoran desert?
impermeable. 2 How is the saguaro cactus adapted to life in the desert?

Vegetation distribution in Death Valley containing chemicals in solution is drawn up by

evaporation and capillary action. As the water
The distribution of vegetation in Death Valley (average
evaporates, salts are deposited on the surface,
rainfall less than 50 mm per year), depends very much
forming thick crusts. These crusts are high in salt,
on ground and soil conditions. These determine the
and so no flowering plants can grow here.
quantity and quality of water supply. Three main zones
Around the edge of the salt pans, however,
can be recognised: the central salt pan, the lower
the ground is not so salty. The groundwater is
sandy slopes and the upper gravel slopes.
still quite high and the ground sandy. In this
The central salt pans occupy depressions, into
zone, phreatophytes (plants with very deep tap
which rainfall runs off from the upper slopes and
roots) grow. Their roots penetrate as far down as
collects. This rain contains dissolved chemicals. As
the water table. The various plants in this zone
it seeps into the ground it raises the water table,
are found in a regular order depending on their
in some places quite close to the surface. Due to
tolerance of salinity. The most salt-tolerant is
the high temperatures of the valley, groundwater

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 Hot deserts

pickleweed, which can tolerate levels as high as occasional overland flow. Once again, a sequence
6 per cent salt (twice as salty as sea water). This can be observed. On the slightly wetter, upper
plant is found closest to the salt pan. Next is slopes, burrow-weed is found. With increasing
arrow weed, which can tolerate 3 per cent salt, and water shortage, creosote bushes are found and,
finally honey mesquite, which can only tolerate a finally, in the lowest, driest gravel, the desert holly
maximum of 0.5 per cent salt content. bush is found.
Between the sandy zone and the valley sides are
the gravel deposits. This is where the xerophytes
(species of plant that have adapted to survive Activities
in an environment with little water) are located. 1 Suggest reasons why hot deserts offer limited
They are too far above the water table to reach opportunities for human activities.
it and survive through being drought-resistant. 2 Explain how plants are adapted to desert environments.
Xerophytes, such as the creosote bush, may receive 3 Explain how human activities have impacted upon
some water from dew, infrequent rainfall and some desert ecosystems.

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 THEME 2

End-of-theme questions
Topic 2.1 Earthquakes and volcanoes

▲ Figure 2.108 Global distribution of earthquakes

Study Figure 2.108, which shows the global distribution c Define the terms ‘focus’ and ‘epicentre’.
of earthquakes. d Using examples, explain the main factors that
a Describe the global distribution of earthquakes. increase the impacts of earthquakes.
b Suggest reasons for the distribution of earthquakes.

Topic 2.2 Rivers

Study Figure 2.109. d Describe the changes in the river valley between point
a Approximately how high is the source of the Jordan A and point B.
stream? e Describe the valley of the Jordan stream and its valley
b How much does it fall in order to reach the at point C.
Waimakariri river? f Outline the likely human activities that might occur in
c How far is the Waimakariri river from the source of this area. Give reasons for your choices.
the Jordan stream?

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 End-of-theme questions

▲ Figure 2.109 Extract from a 1:50,000 map of Arthur’s Pass, New Zealand

Topic 2.3 Coasts

Study Figure 2.110.
a Identify the features A, B, C and D.
b Explain how landform C may be formed.
E c What feature is located at E?
40 d Explain the meaning of the term ‘prevailing wind’.
50
30 e Suggest how landform E may be formed.
40
20

30 10

20

10 D
Solid land
20
B 10 Height in metres
40
C 2m Prevailing wind
from SW

▲ Figure 2.110 Sketch map of coastal features

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 THEME 2

Topic 2.4 Weather Topic 2.5 Climate and natural vegetation

1 Using the data in Table 2.23, plot the data for 16–22 1 Study Figure 2.105 on page 164.
September 2013. Use the same methods as you did for a Comment on the distribution of the world’s hot
Figure 2.95 (for August 2007, see page 155). deserts.
2 State the maximum and minimum temperature of the b Why is it difficult to live in a desert?
7-day period. c How have plants in the desert adapted in order to
3 Work out the mean minimum temperature and the survive?
mean maximum temperature for the 7 days. d How do animals survive in the desert?
4 How much rain fell during the 7 days?
5 Compare the weather of August 2007 and September
2013.

Table 2.23 Daily weather observations for Frankston, Victoria

(Australia), 16–22 September 2013

Date Day Maximum Minimum Rainfall Wind Wind speed Air pressure
Temperature Temperature (mm) direction (km/hour) (mb)
(°C) (°C)
16 September Monday 16.5 12.7 0.4 E 24 1008
17 September Tuesday 16.7 12.4 26.0 ESE 28 1003
18 September Wednesday 16.4 12.7 14.6 WNW 57 999
19 September Thursday 14.4 9.9 19.8 WNW 61 1005
20 September Friday 14.6 10.7 1.0 SW 48 1007
21 September Saturday 17.2 8.1 0.0 NNE 24 1016
22 September Sunday 18.1 9.9 0.0 N 30 1012

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